1. Field of the Invention
This invention concerns a human body signal measuring apparatus for measuring, by radio, such human body signals as myoelectric potentials (EMG=electromyography), oculo-electric potentials (EOG=electro-oculography), cardio-electric potentials (ECG=electrocardiography), and encephalo-electric potentials (EEG=electroencephalography). This body signal measuring apparatus can be employed not only in the field of medicine, but also in the fields of physiology (including kinematics) and psychology (as in studies of attention and arousal). It may also be attached to the driver of an automobile or other moving vehicle to measure the driver's conditions.
2. Description of the Related Art
FIG. 13 is a circuit diagram for a conventional electromyograph. This electromyograph will now be described with reference to this figure.
A conventional electromyograph 80 comprises a pair of detection electrodes (conductor) 821 and 822 for inputting myoelectric potentials, a ground electrode 832, an amplifier 84 for amplifying the myoelectric potentials input from the detection electrodes 821 and 822, a lead wire 861 for inputting the myoelectric potentials amplified by the amplifier 84 to a measuring instrument 88, lead wires 862 and 863 for supplying power to the amplifier 84, a lead wire 864 for supplying a reference voltage to the amplifier 84, and a lead wire 865 for connecting the ground electrode 823 to ground potential.
The detection electrodes 821 and 822 are arranged concentrically in order to measure myoelectric potentials without considering directionality. An IC called the "AD620BR," made by Analog Devices, is used in the amplifier 84. A resistor 841 is connected to this IC for setting the gain. .+-. power supplies 881 and 882 are built into the measuring instrument 88. The .+-. power supplies supply electric power to the amplifier 84 via the lead wires 862 and 863.
Myoelectric potentials may generally be thought of as AC signals referenced to 0 V. Therefore, in order to measure the myoelectric potentials, .+-. power supplies 881 and 882 are needed that are referenced to 0 V. In order to stabilize this reference potential at 0 V, the ground potential is applied to the skin in addition to the detection electrodes 821 and 822. This ground electrode and its connecting lead wire 865 are connected to the amplifier reference terminal. Thus the potential that constitutes the reference for amplification is always 0 V. When a potential variation having a DC component appears in the human body, this becomes a current that flows to ground from the ground electrode 823. Even supposing then that a DC component has been applied to the electrodes 821 and 822, that DC component will be applied to the ground electrode 823 at that time, whereupon the reference potential of the amplifier will also change. Thus the amplifier will perform amplification normally.
An electromyograph such as this is set forth, for example, in Nishimura and Tomita: "Zofuku kino wo motsu kindenikei no shisaku (Electromyograph with amplification functions), " Keisoku Jido Seigyo Gakkai Ronbunshu (Collected Monographs of Automatic Measuring Control Society), Vol. 29, No. 12, pp. 1474-1476 (1993).
With the conventional electromyograph 80, five lead wires 861-865 are required between it and the measuring instrument 88. Hence the movements of the test subject are limited in range to the length of the lead wires 861-865. In other words, the movements of the test subject are limited to simple, easy movements such as will not twist or pull out the lead wires 861-865.
For this reason, an electromyograph that operates by radio signals is desirable. Unfortunately, however, electromyographs that operate on radio signals are not known.
There are disclosures of techniques for transmitting heart rates by radio signals, for example in the laid-open patent applications H2-283354 [1990] and S63-49135 [1988]. These cardiotachometers can measure heart rate without restricting the movements of the test subject, by adding radio signal functions to the electrodes applied to the test subject. Compared to myoelectric potentials or oculo-electric potentials, however, the measurement of heart rate involves large potential differences, wherefore there is little need to consider noise produced by DC components in the human body.
Unlike cardiotachometry, when it comes to measuring such body potentials as heart potential, myoelectric potential, oculo-electric potential, and brain waves, the ground level has to be stabilized. When transmitting by radio, in particular, it is necessary to amplify potential variations in the human body. If the ground level is unstable, the amplifier will suffer saturation or other impairment, preventing the realization of radio transmissions.
In measuring myoelectric potentials, it is desirable to be able to measure potential variation while the test subject is moving. Radio transmission is thus most desirable in order to permit the test subject to move freely and to prevent noise interference due to the shaking of lead wires. In measuring myoelectric potentials, moreover, the smaller the electrodes are made, the more muscle types one can take measurements from. Hence it is desirable to do away with the ground electrode.
In view of these considerations, in designing an apparatus for measuring fluctuations in human body potentials, the following problems need to be resolved.
How to transmit the measured potential variations by radio in order to eliminate the effects of lead wire shaking and allow the test subject to move freely.
How to amplify body potential variations before transmission to facilitate transmitting weak body potential variations by radio.
It is necessary to supply the amplifier with a stable reference potential in order to amplify body potential variations. In order to do this amplifying, however, the DC components in the body and the variations in ground potential in the body need to be absorbed so that the amplifier will only amplify the body potential variations (AC components), thereby permitting a high amplification factor to be sustained.
Even if lead wires are eliminated so as not to restrict the movements of the test subject, the test subject's movements will nevertheless be restricted if the electrometer itself is large and/or heavy. In particular, an electrometer is needed of such size as will not hinder driving operations when attached to the driver of a motor vehicle.
In order to make the electrometer small, the battery needs to be limited to a single cell, and the use of a ground electrode eliminated, while still allowing the amplifier to function stably.
How to prevent crosstalk when multiple electrometers are attached to the test subject in multiple places and radio transmissions are done simultaneously, and how to maintain sufficient precision to permit comparing the potentials detected by the several electrometers, irrespective of differences in the performance of the electrometer amplifiers.